British MPs should pause before authorising a reckless missile strike to punish the regime of Bashar al-Assad, the speaker of Syria's parliament has said.

Mohammad Jihad al-Laham has written an open letter to the House of Commons urging MPs to "turn Great Britain from the warpath" before a crucial debate on military action.

His letter invites a delegation of UK MPs to Damascus to check the conclusions of UN weapons inspectors, calling on them "not to bomb us but to work with us".

"Before you rush over the cliffs of war, would it not be wise to pause? Remember the thousands of British soldiers killed and maimed in Afghanistan and Iraq," he said.

The speaker of the Syrian people's assembly, which backs Assad, spoke out as David Cameron set out the intelligence evidence saying it is "highly likely" the regime used chemical weapons in a suburb of Damascus.

Between 300 and 1,300 people are thought to have died in the attack but the numbers cannot be independently verified.

The prime minister faces an uphill struggle to convince his own Tory MPs to back British involvement in another war.

Resistance from within his own party and Labour has already forced him to back down from asking the House of Commons to vote on an immediate military strike in a debate on Thursday afternoon.

Instead, MPs will vote on the general principle of a "legal, proportionate" military action "if necessary" and get a second vote at a later date before any direct engagement by British forces.

Cameron attempted to win round Conservative MPs at midday by setting out his case in a private meeting, before standing up in the House of Commons at 2.30pm.

The debate on Syria is likely to be heated, with around 30 Tories so far publicly expressing scepticism about the coalition's plan and many more doubters in private.

Labour has tabled a different motion insisting that Britain should only support a military strike if certain conditions are met. These caveats include waiting for the verdict of UN weapons inspectors, trying to get a solution through the UN and ensuring any action is in keeping with international law.

MPs have also been lobbied by a Syrian revolutionary leader, who urged them to "send a message" to Assad that he does not have a green light to use chemical weapons.

Bader Jamous, secretary general of the Syrian National Coalition, called on parliamentarians to stop the Syrian regime from being able to "gas sleeping children with impunity".

He said the revolutionaries were not asking the west to overthrow Assad, but urged Britain to vote for punitive action against the regime for crossing a line.

"The poison gas attack on 21 August was not the first time the regime has used grotesque force against civilians," he wrote. "Since the start of this conflict, it has resorted to tanks, mortars, war planes and ballistic missiles to kill people demanding accountable and competent government. Clearly the approach of the past two and a half years has not subdued the violence or lessened the human suffering. In reality, it has given the Assad regime a green light to escalate … It is clear from the regime's past actions that it will only continue and escalate its violations if it is not stopped."

Jamous said the Assad regime had to be made to understand that "it cannot guarantee its survival by killing people making legitimate demands".

Jamous was elected to the Syrian National Coalition in July as part of a change of leadership within the main western-backed opposition group.